This invention relates to display systems, and more particularly to a display system that uses multiple spatial light modulators (SLMs) to provide color images.
Spatial light modulators (SLMs) are increasingly being used for display applications. SLMs can be characterized by their array of pixel-generating elements, each of which is individually addressable. Image data is coded into a format that permits each pixel-generating element to be addressed for each image. Light emitted from, or reflected by, the SLM is modulated so that each element contributes a pixel of desired brightness to the overall image.
One type of SLM is the digital micro-mirror device (DMD(trademark)), manufactured by Texas Instruments Incorporated. A DMD uses an array of microscopic mirrors that build an image by rapidly switching on and off in response to the image data. The DMD is sometimes referred to as a xe2x80x9cdisplay on a chipxe2x80x9d because of its monolithic formation over a CMOS address circuit. Another type of SLM is a reflective liquid crystal display.
Color images are provided by placing color filters in the optical path and by using one, two, or three SLM in one display system. Additional features of color systems depend on how many SLMs are used. For example, for a three-SLM system, a prism can be used to split and recombine color. Filters deposited on the surface of the prism split the source illumination into red, green, and blue components. Each of these colors is assigned to its own SLM, which reflects monochromatic light back into the prism. The light is then recombined, magnified, and projected onto a screen for viewing.
Three-SLM display systems do provide high brightness levels. However, in the past, such systems have tended to be more costly and less compact than display systems that use one or two SLMs.
One aspect of the invention is an optical unit for a spatial light modulator (SLM)-based color projection display system. A white light source provides a beam of white light. This beam is split into three primary colors, by two or three filters in succession. The number of filters depends on the order in which light is subtracted out of the white light.
Light of each color is delivered to an associated SLM, each of which generates images of that color. Each SLM directs its images to an associated projection lens, which projects the images to a viewing plane. The projection lenses are positioned such that the images from each projection lens converge at the viewing plane.
The color splitting may be accomplished in a least two different ways. In one embodiment, a filter at the face of each SLM transmits the desired color to that SLM. In a second embodiment, a beam splitter associated with each SLM reflects the desired color of light to that SLM. For each embodiment, if the color splitting is done in a certain order, filters are required for only two of the SLMs.
An advantage of the optical unit is that it eliminates the need for a prism to accomplish color splitting. It also uses three narrow band projection lenses rather than a single projection lens that receives the images from all SLMs. These features eliminate many disadvantages associated with using a prism and a single projection lens.
For example, a prismless design is lighter in weight and less costly. The overall size of the optical unit may be reduced because of greater flexibility in the illumination paths.
Each of the three projection lenses may be simple in design. They do not require the color correction and focal length correction that are required when a single projection lens receives images of all colors. Convergence is much simpler in that it may be accomplished by adjusting the projection lenses, rather than by required the images from the SLMs to converge to a single projection lens. The three projection lenses do not have the illumination losses that occur when a single projection lens is used. Thus, all other factors being the same, the system will have improved optical efficiency.